1. Field of the Invention
The present invention relates to a broadcast wave receiving system provided with a tuner that receives broadcast waves and demodulates sound signals and a switching amplifier that transforms the sound signals output from the tuner into pulse-width modulated signals and amplifies the pulse-width modulated signals.
2. Description of the Related Art
Conventionally, a switching amplifier has been known as an amplifying device for amplifying sound signals that realizes a highly efficient acoustic system. This switching amplifier is a device that transforms the sound signals into pulse-width modulated signals (hereinafter referred to as “PWM signals”), and then amplifies the PWM signals to an output level of a loudspeaker.
As shown in FIG. 12, in general, a switching amplifier 100 is provided with a pulse width modulating unit 101 that transforms an audio signal (analog signal) es that has been inputted from an audio device into a PWM signal, an output unit 102 that amplifies the level of the PWM signal, and a filter unit 103 that transforms the amplified PWM signal back to an analog signal (amplified audio signal) by making the amplified PWM signal to pass through a lowpass filter.
The PWM signals Sout and /Sout that are output from the pulse width modulating unit 101 are signals that have been modulated such that a duty cycle of each pulse in a pulse signal is varied according to changes in the level of the audio signal. Further, the PWM signal /Sout is the signal whose phase is inverted from that of the PWM signal Sout. As a circuit that generates the PWM signals, a method of comparing an audio signal with a triangular wave signal and changing an on-period and an off-period (a duty cycle) of the pulse signal according to a magnitude of the level of the audio signal with respect to the triangular wave signal (so called Triangle Wave Comparison) is typically known.
Further, for example, U.S. Pat. No. 7,456,668 discloses a method of generating a PWM signal with a frequency twice as high as a reference clock by generating the reference clock, transforming the level of the audio signal to time in an on-period and an off-period in each cycle of the reference clock, and generating a pulse having the time as an on-period.
This method of generating the PWM signal, as compared to the Triangle Wave Comparison, is not easily affected by a spike noise superimposed over a sound signal or fluctuation of a power-supply voltage, and is especially suitable as a pulse width modulating circuit of a switching amplifier of an acoustic purpose.
The output unit 102 is configured such that two series circuits of semiconductor switches SW1 and SW2 are equivalently connected between terminals to which a pair of voltages with opposite polarities of +Vd and −Vd (where |Vd| is on the order of a dozen) are inputted from a power supply PS. As the power supply PS that supplies the voltages of ±Vd, a switching power supply is also used in order to realize a highly efficient acoustic system. An on-off operation of the semiconductor switch SW1 is controlled by the PWM signal Sout, and an on-off operation of the semiconductor switch SW2 is controlled by the PWM signal /Sout. Accordingly, the PWM signal obtained by amplifying the amplitude of the PWM signal Sout from that of +Vd to −Vd is output from the output unit 102.
Examples of a source of the audio signal es inputted to the switching amplifier 100 include players for CDs and DVDs and tuners for television and radio. However, in the case where the switching amplifier 100 is mounted to an acoustic device that includes a tuner, and audio signals from the television and the radio demodulated by the tuner are also taken as input signals of the switching amplifier 100, the reception operation by the tuner is often interfered by a frequency for turning on or off the semiconductor switches SW1 and SW2 of the output unit 102 of the switching amplifier 100 and by frequency for turning on or off a semiconductor switch within a switching power supply PS. Accordingly, these frequencies (hereinafter these frequencies are referred to as “operating frequencies”) are required to be selected so as not to interfere with the reception frequency of the tuner.
On the other hand, according to the switching amplifier 100, increasing the frequency of the reference clock of the pulse width modulating unit 101 and reducing the length of the cycle decreases a sampling cycle of the audio signal es that carries out the pulse width modulation. As a result, while the playback quality is improved in terms of the sampling number, the modulation is not carried out properly, and the playback quality is not improved in terms of modulated strains. In other words, as a ratio of times of rising and falling of the PWM signal with respect to the switching cycle increases, and a dynamic range of the sound signal es in which the pulse width modulation is normally carried out becomes narrow, and selection of the frequency of the reference clock is restricted to a certain extent in view of the playback quality.
Further, as the semiconductor switches SW1 and SW2 of the output unit 102 are required to be turned on or off by the PWM signals Sout and /Sout without fail, the selection of the frequency of the reference clock is restricted to a certain extent in terms of responsivity of the semiconductor switches SW1 and SW2.
The operating frequency of the switching amplifier 100 corresponds to the frequency of the PWM signals Sout and /Sout output from the pulse width modulating unit 101, and the frequency is twice as high as the frequency of the reference clock. The frequency of the reference clock is selected in view of the playback quality and the responsivity of the output unit 102, and consequently, an operating frequency fc of the switching amplifier 100 is generally within a range from 300 kHz to 500 kHz.
On the other hand, in AM radio broadcasting within Japan, band frequencies of the carrier waves are assigned to a range on the order of 500 kHz to 1700 kHz, and for example, the carrier waves with 9-kHz pitch and of 124 channels within a range from 522 kHz to 1629 kHz are available.
In particular, since a reception circuit of superheterodyne principle is employed for AM tuners, frequencies that should be avoided so that the operating frequency fc and its harmonic frequency n×fc (where n is an integer of n=2, 3, . . . ) of the switching amplifier 100 do not become interfering waves are not only the carrier frequency f0 (the carrier waves of 124 channels) in a range from 522 kHz to 1629 kHz, but also an intermediate frequency fIF (=455 kHz) and an image frequency fimg (=f0+2×fIF) that is a mirrored image of the carrier frequency f0 centering the intermediate frequency fIF. It is extremely difficult to select a single frequency as the operating frequency fc and use this frequency in a static manner without causing any interference to all the frequencies.
For example, since not all of the 124 channels are used as the carrier waves in all regions, it is possible to ease the condition for the selection and facilitate the selection of the operating frequency fc by setting a condition that no interference is caused to the carrier frequency f0, the intermediate frequency fIF, and the image frequency fimg that are used in each region. However, this method is not practical, as it is necessary to change the operating frequency fc according to selling areas of products, hindering standardization of management in manufacture and manufacturing processes.
Further, it is conceivable that a method of configuring the reference clock of the switching amplifier 100 with a variable frequency clock, and automatically setting the clock frequency of the reference clock that does not interfere the reception of the reception frequency every time when the reception frequency of the tuner is set. However, this method poses problems that the configuration of the reference clock becomes complicated, and that, as it is necessary to select the clock frequency of the reference clock based on the condition for the selection of the frequency for ensuring the playback quality required for the switching amplifier 100 and the responsivity of the output unit 102, the circuit for carrying out the selection process also becomes complicated.
In the case of the switching power supply PS, the operating frequency (switching frequency) often becomes lower than the operating frequency fc of the switching amplifier 100, in general, on the order of several dozen kHz. Similarly, in this case, it is necessary to select the operating frequency so that the operating frequency and the harmonic frequency do not interfere with the carrier wave f0, the intermediate frequency fIF, and the image frequency fimg. Accordingly, the above problems also apply to the selection of the operating frequency of the switching power supply PS.
It is also conceivable that the multiple clock frequencies and the switching frequencies are previously set such that poor reception occurs to a part of a large number of carrier frequencies included in the broadcast wave band that the tuner receives but not to the same carrier frequency, and the clock frequencies and the switching frequencies are switched according to the reception frequency of the tuner.
However, for example, in the case of the clock frequency, when the carrier frequency of the receiving station of the tuner is changed in the state in which the sound signal from the tuner is amplified by the switching amplifier 100, if the clock frequency of the reference clock of the switching amplifier 100 is changed along with this change, the operation of the switching amplifier 100 may possibly become unstable as the frequency of the PWM signal changes dramatically when switching the clock frequency. In the case of the switching power supply, the output voltage is fluctuated and the switching power supply may possibly become unstable.
Therefore, in the case of the switching amplifier, it is preferable that an amplifying operation of the switching amplifier 100 is temporarily suspended when switching the clock frequency, and the amplifying operation is resumed after switching the clock frequency. However, carrying out such a process poses problems that it could give a feeling of strangeness to the user if the amplifying operation of the switching amplifier 100 is suspended or not depending on the presence of the change in the clock frequency when the user has changed the receiving station, and that convenience of the broadcast wave receiving system is impaired. On the other hand, in the case of the switching power supply, it is not preferable to stop outputting in mid-course.